The present invention relates to a cam for use in an engine valve system of an internal combustion engine, and more particularly to an engine cam which actuates an engine valve system equipped with a hydraulic valve lash adjuster.
It is common practice in the internal combustion engine art to employ an overhead cam arrangement to increase engine speed and operating efficiency. An overhead camshaft design increases the efficiency of valve train operation since the cam lobes bear directly upon the rocker arms which actuate the intake and exhaust valves, rather than push rods which are employed in conventional valve train arrangements. The removal of the push rods results in a reduction of the reciprocating weight of the engine, thereby increasing the potential RPM of the engine. To ensure quiet operation as well as a relatively long operative life, the tolerances between the various moving parts of an overhead camshaft-type arrangement must be kept within close tolerances. Any increase in the lash between the cam lobes and the follower surfaces on the rocker arms will result in noisy engine operation as will as increased wear of the moving parts.
The use of a valve lash adjuster to reduce the clearance between the arm and the cam, resulting in the elimination of the noisy lash, has become increasingly popular. The lash adjusters operate off of the pressurized engine oil, and the force exerted by the oil on the end of the adjuster plunger increases the mechanical leverage of the rocker arm and forces the cam lobes into contact with the rocker arm at the rocker contact surface. Therefore, the adjuster does not permit any clearance between the rocker arm and the cam.
A valve seated in an inner wall of a cylinder head, is downwardly actuated by the valve stem, which transfers the motion of the cam and rocker arm. This downward actuation is directed against the force of the valve compression spring which upwardly biases the valve toward the valve seat. Consequently, the potential loose contact which may occur between the valve and the valve seat, results in the air-fuel mixture in the combustion chamber of the engine flowing through an exhaust/intake port at an unscheduled moment.